1. Field of the Invention
The present invention relates to a plasma processing apparatus that is applied to a fabrication process of a semiconductor device such as a thin-film transistor (TFT) or a metal oxide semiconductor device (MOS device), a semiconductor device such as a semiconductor integrated circuit device, or a display device such as a liquid crystal display device.
2. Description of the Related Art
Conventionally, a plasma apparatus, such as a parallel-plate radio-frequency plasma processing apparatus or an electron cyclotron resonance (ECR) plasma processing apparatus, has been used to execute a plasma process, such as film deposition, surface modification or etching, in a process of fabricating, for instance, semiconductor devices and liquid crystal display devices.
In the case of using radio-frequency waves for excitation of plasma, radio-frequency waves are radiated from an antenna, and electrons are accelerated by radio-frequency electromagnetic field, thereby maintaining the plasma. The electromagnetic field in the vicinity of the antenna includes three electromagnetic field components, that is, a radiant wave, an induction electric field and a static electric field, which decrease in proportion to f−1, f−2 and f−3, relative to a frequency f. As the frequency for exciting the plasma becomes higher, the induction electric field becomes dominant over the static electric field. In a microwave range with higher frequencies, the radiant wave becomes dominant. Because of this frequency dependence of electromagnetic fields in the vicinity of the antenna, when plasma excitation is executed with frequencies from the HF range to the VHF range, an electrostatic-coupling (capacitive-coupling) type parallel-plate apparatus is used. On the other hand, in the case of microwaves, an apparatus, which is configured to execute plasma excitation by electromagnetic waves radiated from the antenna, is used.
The parallel-plate plasma processing apparatus, however, has such a problem that the plasma density is low and the electron temperature is high. In addition, the ECR plasma processing apparatus requires a DC magnetic field for plasma excitation, and there arises such problems that the apparatus becomes complex and large in size and the processing of a large-diameter semiconductor substrate is difficult.
To solve the above problems, there has been proposed a microwave plasma processing apparatus that requires no magnetic field for plasma excitation and is capable of generating a high-density plasma with low electron temperatures (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 7-142194). In this case, electromagnetic waves are radiated through a dielectric window into a reaction chamber where a plasma process is performed. In general, the dielectric window passes microwaves and maintains a vacuum state in the reaction chamber by means of, e.g. an O-ring, and the dielectric window is exposed to a processing plasma. Thus, the dielectric window needs to be formed of a material that has good microwave introduction characteristics, a mechanical strength and heat resistance against a mechanical stress due to an atmospheric pressure and a thermal stress due to heat from the plasma, and a corrosion resistance to a plasma that is produced from, e.g. a fluorine-based gas that is used for the plasma process and cleaning. Various proposals have been made in order to fabricate a dielectric window with such characteristics.
For example, it is proposed that the thickness of a dielectric window is set at an n/2-number of times or n-number of times (n=an integer) of a half-wavelength of microwaves so as to take advantage of an interference effect of incidence and reflection, thereby to achieve good microwave introduction characteristics, that is, stable discharge of plasma (see, e.g. Jpn. Pat. Appln. KOKAI Publications Nos. 10-255999 and 10-199698). Moreover, to achieve a resistance to mechanical stress and thermal stress, there have been proposed a method in which the thickness of a dielectric window is set at a very great value and a method in which a dielectric window is provided only at a partly formed aperture portion (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 7-272897). Besides, in order to improve the heat resistance and the corrosion resistance at the time of using fluorine-based gas, an attempt has been made to form the dielectric window of a ceramic material such as aluminum nitride (AlN) or alumina (Al2O3) or of a material coated with such a ceramic material (see, e.g. Jpn. Pat. Appln. KOKAI Publications Nos. 9-95772, 10-275524, 8-279490 and 9-102488).
In recent years, studies have been made as to whether the microwave plasma processing apparatus is applicable to processing of semiconductor substrates with greater diameters or to larger-area substrates for LCDs, etc. In order to realize processing of larger-area substrates, it is necessary to further improve the mechanical and thermal strength of the dielectric window. In order to clear the problem, there has been proposed a technique wherein metallic beams are provided on a cover member to which a dielectric window of a process chamber is to be attached, and the dielectric window is divided (see, e.g. Jpn. Pat. Appln. KOKAI Publications Nos. 8-250477, 8-274065 and 10-92797).
As methods for generating a large-area plasma, there have been proposed a method of using a plurality of microwave waveguides (see, e.g. Jpn. Pat. Appln. KOKAI Publications Nos. 8-316198 and 8-31593), and a method in which a microwave introducing section is configured to have an annular structure with slots, thereby to realize reduction in size of the apparatus while enabling large-area processing (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 11-121196).